The invention relates to a method for the manufacture, in a continuous process, of a flat substrate for a printed wire board (PWB) from a fibre-reinforced matrix, in which process at least two moving layers of threads of reinforcing fibres are employed and fibres are disposed in at least two crossing directions, after which the layers of threads provided with matrix material are passed through a preferably heated laminating zone, such as a double belt press, and the laminate is provided on one or on either side with means for depositing electrically conductive traces, e.g., for deposition on the laminate of a copper foil that can be partially removed again by etching. The invention further comprises a printed wire board having favourable properties.
Such a process is known, int. al., from EP 0 231 737 A2. For the layers of threads of reinforcing fibres use is made in said known process of fabrics or non-wovens in which the fibres are interlinked in the conventional manner and have a large number of curves and twists.
Although passable results can be obtained with the known method, there are certain drawbacks to the use of reinforcing fibres in the form of woven fabrics or non-wovens, which drawbacks are augmented in the case of printed wire boards of higher interconnection density and/or smaller thickness. Particularly when printed wire boards are employed which are deposited on a thin substrate composed of, say, glass cloth impregnated with epoxy resin, the following drawbacks may become manifest: high production costs to manufacture the thin fabric, a relatively large number of process steps, a low production rate, an existing production process that is hard to automate, and a comparatively mediocre surface quality of the substrate, as well as at times varying properties in two mutually perpendicular (X- and Y-) directions parallel with the plane of the substrate. Another drawback to a conventional substrate of thin woven fabrics is the relatively limited content of fibre material with a comparatively low linear thermal coefficient of expansion (TCE). The corresponding high content of matrix material, such as epoxy resin, of comparatively high TCE generates linear thermal coefficients of expansion (TCE-X, Y, Z) of the conventional substrate that are high as compared with the TCE of the components to be mounted on it. The modulus of elasticity and the strength are not as high as is feasible either, nor is the dimensional stability sufficient for certain applications. Conventional printed wire boards containing non-wovens as reinforcing material will generally exhibit the same drawbacks as when woven fabrics are employed. The non-wovens are of the type described in U.S. Pat. No. 4,547,421, the fibres or filaments lying substantially in one plane in random twists and curves and being thermally, chemically, and/or mechanically interlinked. The term non-woven, whenever it is used in the description and the claims, refers to a product of the aforementioned type.
EP 0 215 392 B1 also describes a continuous process for the manufacture of substrates for printed wire boards, in which the reinforcing fibres used are likewise in the form of glass fabrics impregnated with epoxy resin.
EP 0 326 577 (WO-88/01938) describes a discontinuous system for the manufacture of a substrate for printed wire boards, in which layers of reinforcing threads which cross at 90.degree. are manufactured with the use of a flat square form, about which reinforcing threads are wound as it is rotated about two axes of symmetry in succession.
In U.S. Pat. No. 4,587,161 a manufacturing process for printed wire boards is described in which the substrate is made up of a mixture of polyester resin and epoxy resin reinforced with a glass fibre mat or with short fibres distributed into the resin.
EP 0 374 319 A1 describes a discontinuous process for the manufacture of multilayer printed wire boards comprising the steps of: forming a plurality of layers of a composite material by lay-up of a fibre tape, the fibre tape being a single layer of fibres coated with a resin and heated to a semi-cured stage.
In WO 89/00216 a process is described for forming a nonwoven layer of two groups of perpendicularly crossing yarns. With the weft yarns under tension, said layer is introduced into a layer of settable material.
WO 83/02085 describes a process in which a laminate composed of a number of layers of perpendicularly crossing threads which do not form a woven fabric is impregnated with a thermoplastic synthetic material. From the resulting fibre-reinforced sheet there may be formed, after heating, various fibre-reinforced end products that may be closed or not.
In EP 0 153 060 A3 a process is described for the manufacture of a flexible reinforced sheet material which may be employed in the same end uses as tarpaulin or as a fabric for inflatable structures or as a robust wrapping material. The manufacturing process comprises the forming of several layers of crossing, non woven layers of threads. These are introduced into a layer of bonding material formed on a moving conveyor belt.
GB 1 364 076 describes a structural laminated fibre-reinforced plastics sheet material composed of a number of UD prepregs of, say, glass fibres or carbon fibres in a thermosetting resin disposed on either side of a datum plane. In superposed prepregs the fibres are at different angles to each other. The prepregs package may be coated on the outer sides with a sheet of titanium.
IBM Technical Disclosure Bulletin Vol. 22, No. 5, October 1979, page 1799 describes a laminate for printed wire boards with a low dielectric constant because the matrix of epoxy resin contains a filler in the form of hollow glass microspheres having a diameter of 20-200 .mu.m and a wall thickness of 0.5-2 .mu.m.
GB 1 043 893 describes a continuous process for the manufacture of a non-woven. First, a central double layer of "weft threads" is deposited and subsequently covered on either side with a layer of "warp threads". The non-woven is next passed through an adhesive bath. The products can be used for producing table napkins, towels, tablecloths, etc., which articles can be thrown away after use.
In actual practice there is a tendency toward constructing more compact electronic equipment. This means that, increasingly, also printed wire boards will have to be manufactured which have a higher interconnection density and satisfy more stringent quality requirements. Also because of this trend it is possible that the use of conventional substrates or boards containing reinforcing fibres in the form of woven fabrics or non-wovens or short fibres will increasingly meet with objections on account of the drawbacks listed above.
The invention has for its object to provide a process of the type mentioned in the opening paragraph in which said drawbacks have been overcome.